Simple fractures of bones are readily treated by bringing the fracture surfaces together and holding them in the desired orientation with respect to one another through the use of splints, casts and the like. Bones in general have dense outer, strong cortical portions and interior, non-cortical portions that may include cancellous bone.
Comminuted fractures and fractures involving the breakage of a bone into numerous bone fragments are especially difficult to deal with since one must attempt to reposition each bone fragment in an orientation relative to each other bone fragment such that the fragments may knit together properly. For this purpose, physicians have often used metal plates that attach to the outer cortical surfaces of the bones and which utilize bone screws to hold the bone fragments in the desired position.
Another method for treating such fractures involves the use of cerclage procedures in which a wire is, in effect, wrapped about a broken bone to hold the fragments in place, the cerclage wire occasionally penetrating through the bone. Reference is made to Johnson et al., U.S. Pat. No. 4,146,002. Yet another method taught in Berger, U.S. Pat. No. 5,658,310, involves anchoring the balloon portion of a balloon catheter in the medullary cavity at one end of a long bone having a transverse fracture, and stretching the remaining portion of the elastic catheter across the fracture interface within the bone to maintain the fracture interface in compression. It would appear that unless the elastic catheter traverses the precise center of the bone at the fracture site (which may be difficult to accomplish, considering the bowed or curved nature of most bones), compressive forces will be uneven across the fracture site. That is, the compressive forces on the side of the bone nearest the catheter will be greater than the compressive forces on the opposite side of the bone, generating an unwanted bending moment across the fracture site.
With cerclage procedures, one must entirely encircle a bone in order to hold the bony parts together. Surgical procedures used to mount bone plates and cerclage elements to a bone often require supportive tissue that is normally joined to the bone to be cut from the bony tissue to enable direct visual access to the bone.
Procedures using bone plates and cerclage elements often tend to interrupt blood flow to the damaged bone fragments, thus hindering the healing process. Moreover, the use of bone plates and cerclage elements, particularly the former, can lead to stress shielding of the fracture site. While Wolff's Law teaches that bone growth is stimulated when stress is applied, continuous, excessive pressure applied to a bone may cause unwanted resorption of bone at the pressure site. In order to promote healing of bone fractures, the fracture surfaces that are brought together during reduction of the fracture should be subject to cyclic or periodic compressive forces so as to stimulate the growth of new bone across the fracture interface without causing bone resorption. When a fracture interface is immobilized, as by a cast, the bone material that is deposited at the fracture interface may have a collagen fiber matrix that is random rather than aligned with the fiber matrix of bone on either side of the fracture, the healed fracture interface being weaker in tension than bone on either side of the interface.
Some bone fractures result in the production of many bone fragments, and proper reduction of the fracture requires the fragments to be carefully reassembled next to each other with their fracture surfaces in contact. Bone screws and bone plate devices commonly are used for this purpose. Using bone screw techniques, two bone fragments may be joined together, and these two fragments as a unit may be moved into approximation with a third fragment and joined to it, and so on. Fragments that are thus joined together by rigid screws cannot move with respect to other fragments, and mismatching of the fracture surfaces as the first several fragments are joined together can have a compounding effect, causing mal-union or non-union of fracture surfaces and resulting in far less than perfect bone fragment assembly and healing.